Rapid progress in the field of micromachining has been accompanied by marked improvement in the performance of reconfigurable logic circuits such as an FPGA (Field Programmable Gate Array) and CPLD (Complex Programmable Logic Device). Further, a reconfigurable processor, which makes it possible to reconfigure a logic circuit during system operation, has been put into practice in recent years.
A reconfigurable processor has a plurality of circuit configuration planes and is characterized in that it can be reconfigured in a time (one to several clocks) much shorter than that an FPGA or CPLD.
The circuit packing density per unit is rapidly increasing in these reconfigurable processors. In addition, the processing functions demanded also are increasing. As a result, there has been a great increase in area and in operating frequency and power consumption is continuing to rise.
Further, the specification of Japanese Patent Application Laid-Open No. 2004-022724 describes that in a case where the temperature of an FPGA detected by a temperature sensor exceeds an allowable temperature, an operating frequency lower than the present operating frequency is set and the FPGA is reconfigured in accordance with the operating frequency set.
An increase in power consumption brings about an increase in heat-release value at the time of operation. This has a major influence upon the operating characteristics of the transistors that construct the reconfigurable processor and is a cause of unstable circuit operation and malfunction.
In general, a method of cooling an LSI package by providing the exterior of the package with a cooling fan is used as means for dealing with the heat evolved by an LSI circuit. A method employed recently employs a water cooling technique in which heat is removed from the LSI package by causing a cooling liquid circulated as by a pump to come into contact with the outer side of the LSI package.
However, these external cooling methods are disadvantageous in terms of operating noise, cost, power consumed by the cooling equipment, area required for installation and degree of design freedom.
Further, a method of lowering circuit operating frequency to suppress power consumption per unit time and prevent a rise in temperature also is available as a heating countermeasure other than cooling by external equipment. In this case, however, it goes without saying that operating speed is sacrificed.